The invention relates to a process for the production of aromatic amines by the catalytic hydrogenation of aromatic nitro compounds.
Aromatic amines are important intermediates, which have to be produced at low cost and in large quantities. Production plants for aromatic amines are therefore generally constructed for very large capacities. The hydrogenation of nitro-aromatics is a strongly exothermic reaction. The dissipation and energy utilisation of the heat of reaction is therefore an important factor in the production of nitroaromatics.
Various reactors are suitable for the gas phase hydrogenation of nitroaromatics. Thus, for example, in U.S. Pat. No. 3,136,818, a process is described in which the reaction is carried out in a fluidised bed. The effective dissipation of heat in this method is impeded by problems resulting from the non-uniform residence time distribution (break-through of the nitroaromatics) and from catalyst abrasion.
Other processes use stationary catalysts in fixed beds. With this arrangement, the reaction can be performed with a very narrow residence time distribution and avoiding the problem of catalyst abrasion. By operating the fixed bed reactor adiabatically, problems of heat dissipation can be avoided. A process of this type, which is distinguished by a simple construction and easy scalability of the individual pieces of apparatus, is described in, for example, EP-A 0 696 574. To keep the adiabatic temperature increase within limits, however, very large gas streams have to be fed into the circulation in the adiabatic operating method.
In DE-A 28 49 002, a process for the reduction of nitro compounds in the presence of stationary, palladium-containing, multi-component supported catalysts in cooled shell-and-tube reactors is described. The contact consists substantially of 1 to 20 g of palladium, 1 to 20 g of vanadium and 1 to 20 g of lead per litee of α-Al2O3. It has proved advantageous here if the active components are present precipitated as close as possible to the surface of the catalyst in a very sharply defined zone and no active components are contained in the inside of the support material. One disadvantage in the gas phase hydrogenation described in DE-A 28 49 002 is the low specific loading of the catalysts. The loadings quoted are approx 0.4 to 0.5 kg/(1·h). The loading is defined here as the quantity of nitroaromatics in kg per litre of catalyst bed passed through within an hour. Associated with the low catalyst loading is an unsatisfactory space-time yield in industrial-scale processes for the production of aromatic amines. Moreover, the selectivities at the beginning of a run period are distinctly lower than towards the end, which leads to losses of yield and problems in working up the crude product.
The loading of the catalyst can be increased in isothermally operated reactors only if the heat released during the reaction can be efficiently dissipated. In WO 98/25881, the use of inert materials to dilute the catalyst bed in the production of aromatic amines is described. As a result of the dilution, the reaction zone is extended and thus the area available for heat exchange is enlarged. With this method, the hot-spot temperature can be reduced or the possible nitroaromatics loading increased with a constant hot-spot temperature. As a result of the dilution, however, the service life of the bed decreases. In the example quoted in WO 98/25881, the productivity of the diluted bed was markedly lower than the productivity of the undiluted bed because of the short service lives, despite a higher loading.
In another process variation, the hydrogenation of nitroaromatics is performed in thermostatically controlled shell-and-tube reactors. Supported copper or palladium catalysts, among others, are used as catalysts. In GB-A 1 452 466, a process for the production of aniline in a thermostatically controlled shell-and-tube reactor using a supported copper catalyst is described. To complete the conversion, a catalyst bed is used there as an adiabatic secondary reactor.
In DE-A 199 31 902, a process for the production of monolithic oxidation catalysts and their use in the gas phase oxidation of hydrocarbons is described. Here, a monolithic honeycomb catalyst is connected as an adiabatic reactor downstream of the isothermally operated main reactor.
The object of the present invention is to provide a process for the production of aromatic amines by the catalytic hydrogenation of aromatic nitro compounds, which can be carried out on an industrial scale and, compared with the processes known from the prior art carried out in shell-and-tube reactors, makes possible a higher space-time yield and longer service life.